Magnetic recording with patterned media was proposed to increase the data density in of hard disk drives. In patterned media hard drives, magnetic material on a disk is “patterned” in small isolated islands, each island being separated from neighboring islands by a non-magnetic region. When data of a digital signal are written to a disk bearing patterned media, the transitions between bits of the digital signal should occur only between islands, and the individual bits of data should be written only to the islands. To assure such operation, a write head must be precisely aligned with the islands of the patterned media so that the data are correctly written to the patterned data islands. Moreover, such operation requires a relatively high level of synchronization of a write clock signal with the motion of the data islands as the disc rotates, compared to the level of synchronization required for writing data to non-patterned magnetic media.
A conventional magnetic recording hard disk with non-patterned magnetic media, has a set of concentric data tracks. Each data track is divided into a plurality of circumferentially spaced sectors. The beginning of each sector is marked by a servo header (magnetization pattern) that contains a synchronization field (a preamble). In use, the servo headers are read by a magnetic read head and the read information of the servo headers is used by way of servo electronics for various purposes. More particularly, the preambles are followed by positioning information used by servo electronics to move the head to desired data tracks and to keep the head centered on the various data tracks as required. The servo preambles and the following positioning information are written to the disk once, typically during the manufacturing or formatting of the disk, and are not be erased during normal operation of the disk drive. A synchronization system for synchronizing operation of disk drives with patterned media, generally use the servo preambles for adjustment of the phase and the frequency of a write clock signal used to effect the writing of data to the disk.
In the prior art, a main problem in the design of the synchronization systems, has been the construction of a synchronization system which is capable of establishing and maintaining “correct” phase and frequency of the write clock signal (referred to below as the “write clock”) over the entire interval between adjacent servo preambles. The term “correct” is used in the sense that “correct” write clock signals for application to a write head, have a frequency and phase synchronized with the rotation of a disk which is appropriate to effect writing of data bits only to the magnetic islands, while inter-bit transitions occur between the islands, on a data track. There are several US patents representative of the prior art, which suggest different approaches to that problem.
A method of detecting write clock synchronization error (resulting in deleted or inserted bits in a read back signal) is described in U.S. Pat. No. 8,035,910. In the '910 patent, an arrangement is disclosed that utilizes both a read head and a write head. In that arrangement, the read head trails the write head as the disk rotates. The write head writes a symbol and the read head reads a just-written symbol. The symbol that has been read is compared with the symbol that was to have been written, and when the read symbol does not match the written symbol, synchronization error is deemed to have occurred.
In U.S. Pat. No. 7,864,471, it is proposed to correct synchronization errors by use of error-correcting codes. A short inner modulation code transforms synchronization error into bit substitution errors, which are removed by a longer outer error-correcting code.
A disadvantage of both the '910 patent and the'471 patent is connected to the fact that immediately before and after synchronization error, there is phase misalignment in the write clock signal with respect to the islands of the patterned media. As a result, the bit error rate rises in the corresponding time intervals and that rise cannot be tolerated.
U.S. Pat. No. 7,911,724 describes a write clock synchronization apparatus, where a needed phase adjustment accuracy is provided by inserting additional synchronization fields between successive servo preambles. A calibration signal is written to each of the synchronization fields, where the calibration signal has a frequency that is offset from the nominal island's frequency of a patterned media. Reading the calibration signal and mixing it with a reference periodic signal creates a possibility to determine the needed phase correction of the write clock. A disadvantage of such synchronization apparatus is the necessity of additional synchronization fields, insertion of which reduces the area of the disk available for data recording.
A prior art synchronization apparatus that is free of the disadvantages of the patents described above, is proposed in U.S. Pat. No. 7,675,703. The apparatus of the '703 patent includes a counter for counting the integer number of write clock cycles between successive servo preambles, and further includes a phase detector for measuring the phase difference between the servo preamble and a reference signal. The total time between successive servo preambles (servo sector length) is determined by combining the counter reading with the measured phase difference. The calculated servo sector length is compared to a nominal value and the difference is used as an indicator of the write clock timing (i.e., synchronization) error. A Kalman filter is used as a state estimator. The Kalman filter receives the timing error as an input value and calculates timing parameters of the write clock according to a state estimation algorithm. The calculated timing parameters proceed to write clock control logic which establishes correct phase and frequency of the generated write clock.
However, one of the factors that affect the accuracy of a write clock synchronization apparatus, is the instability of the hard disk angular speed, or rate of rotation. Changes of the angular speed during one revolution are primarily due to eccentricity of the circular data tracks with respect to the actual center of rotation (“repeatable runout”). By keeping a history of write clock frequency error for a number of disk revolutions, the synchronization apparatus can determine the repeatable error components and compensate them during write operation. In contrast to the repeatable run out, the slow fluctuations of the spindle speed of a disk drive, with changes from one revolution to another, are of erratic nature and cannot be compensated.
The synchronization apparatus described in '703 patent uses the difference between the measured sector length and its nominal value as a timing error—a source for the needed correction of write clock phase and frequency. Since the measurement of the servo sector length inevitably contains an error due to changes of the angular speed, the instability of the disk angular speed makes detrimental impact on the apparatus operation, constraining the achievable synchronization accuracy.
The goal of present invention is to create a write clock synchronization apparatus that is unaffected by the hard disk angular speed instability and, in that way, to enhance the accuracy of the apparatus operation.